Placing of Sensors in HVAC Systems

Obviously, the sensors have to be installed in a correct and representative place in the process. Determining an optimal installation of sensors for mea­suring environmental conditions in large halls is not a simple task. Many dif­ferent factors have to be taken into account. The main place where a certain climate is to be maintained is given priority. Secondly, the influence of infiltra­tion and radiation from surrounding surfaces must be considered.

The influence of airflows from ventilating systems must also be consid’ ered. Processes using mediums of different physical qualities when mixed will have separation into different layers. Transmission of energy between molecules in flowing mediums takes place in the direction of the velocity. This strengthens the separation into parallel layers. The level of fluid in containers and tanks is due to stratification of horizontal temperature lay­ers, while airflow after batteries, heat-recovery systems, and humidifiers or dehumidifiers will separate into parallel layers.

Factory-constructed ventilation systems are constructed in a compact manner. Frequently, due to the smaller-space installation, inspection and maintenance of the sensors can be difficult. In ducts, deviations from the real

Values of measurements occur due to the sensor or receptor being exposed to moisture or heat or cold radiation from the surroundings. Choosing the incorrect sensor or wrongly positioning it can lead to hunting. The hunting influences the process and makes regulation difficult.

To reduce pollution contamination, sensors or receptors installed m ducts or pipes are placed and installed after the filtration of the air or fluid. Sensors in pipes must be installed primarily in bends and in the direction against dynamic flow. At least two-thirds of the sensor pocket has to be in contact with the fluid. If the pipe diameter is less than the sensor length, the pocket can be installed at an angle less than 90° against the direction of flow to provide the necessary contact area.

Depending on the accuracy required, the pressure difference across com­ponents in a flowing gas or fluid is measured by sensors for pressure differ­ence. The quantity of flowing medium can be determined by measuring the flow velocity in pipes or ducts. Sensors for measuring the velocity of gases or liquids are dependent on the nature of the flow if it is laminar. If accurate measurements of the velocity of gas or liquid are required, the sensors have to be installed in straight runs of the pipe or duct. This part of the system should have a straight section longer than five times the diameter of the pipe or the duct. Both velocity and pressure differences can be transformed into respective values by mathematical formulas in the function modules of the DDC controller.

In buildings that are divided into zones with a central heating system, it is common to change the water temperature depending on the outdoor tem­perature. In this example a function called feed-forward or compensating is used. Figure 9.55 shows how the water temperature changes as a function of the outdoor temperature.

Sensors TS 1-2-4 regulate the batteries for heating and cooling in a se­quence to achieve the required temperatures (Fig. 9.56). Regulating valves for heat recovery are controlled by a frequency converter RC1 for the pump motor. When a greater output is required from the heating battery, the pump motor speed increases before the valve MV2 opens. If the extract tempera­ture is lower than the outdoor temperature, the speed of the pump motor in­creases before valve MV1 opens. To avoid ice formation at low outdoor temperatures, the sensor TS7 operates on a lower limit, depending on the de­mands of the battery in the exhaust.

Saturated air is difficult to measure accurately; a deviation of 3-5 K must be accepted. In some air conditioning systems, very humid air may condense on surfaces below the dewpoint temperature of the air. Poorly in­sulated ducts containing humid air can cause serious problems. See Fig. 9.57. ‘

The airflow rate to each room is controlled by a damper operated as a function of the room temperature. Sensors that measure the CO^ concentra­tion in the room air provide an extra mode of control, in addition to that of air temperature.

The speed of the fans is controlled by sensors PS1 and PS2 and fre­quency converters RC1 and RC2 (Fig. 9.58). The sensors measure the differ­ence in pressure between duct and atmosphere outside and maintain constant pressure in the ducts.

Placing of Sensors in HVAC Systems

Radiator

HEATING SYSTEM FLOW DIAGRAM

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Placing of Sensors in HVAC Systems

FIGURE 9.55 A central heating system supplying low-pressure hot-water radiators in a building.

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Placing of Sensors in HVAC Systems

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Placing of Sensors in HVAC Systems

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FIGURE 9.57 A ventilation system for controlling the air temperature by humidifying and dehumidifying the air.

 

Placing of Sensors in HVAC Systems

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